The C2 and C2N have different viewing hole locations in the pressure plate to line up the starting arrows. If you are starting with a used rollfilm adapter, it's not a bad idea to take an expired roll of film, load the back, pull the darkslide and mark the outside of each frame with a fine pointed felt tip (or whatever will write on the film) as you wind it through.

If anyone has tampered with the rollfilm adapter, you may be able to (or may have to) adjust the point where you stop the film and set the counter dial (if it is working correctly, you can line the starting arrows up in the "window" in the pressure plate. But only "if".

And you might want to KNOW that the frames are falling correctly on the film before shooting once in a life time photos. And if the counter starts so far off that it isn't usable, you may want to return it to the seller.
bob